Prior art systems for achieving composting of solid waste and sewage sludge typically employ one or more multi-stage digesters in which material being treated undergoes staged microbial decomposition. The conventional digester is divided into two or more compartments or stages and during material processing is rotated while air is circulated through the digester at controlled rates under predetermined conditions in a flow direction counter to the material flow. The climate in each stage is maintained to achieve the optimum development of the type and species of microorganism predominant in that stage. Spent air is vented from the digester to the atmosphere as needed to maintain optimum climatic conditions in each of the operating stages. Temperatures are kept below 150 degrees F. to ensure the maximum rate of composting. Typical of such prior art systems and methodology of operation are those set out and described in U.S. Pat. Nos. 3,245,759 and 3,138,447 assigned to the assignee of the present invention, the teachings of which are hereby incorporated by reference.
The method and apparatus for manufacture of compost described in those patents is designed to produce aerobic decomposition of organic waste materials by maintaining within the apparatus in which the method is carried out, conditions suitable for optimum propagation of the different types of aerobic bacteria on which such decomposition depends. The apparatus comprises a digester in the form of a cylindrical drum mounted for rotation on an axis which is slightly declined towards the discharge end relative to the horizontal. The interior of the digester is divided into a series of compartments or chambers by a plurality of transverse partitions spaced along the axis of rotation. Each partition is provided with transfer buckets which are selectibly opened and which when opened, transfer material from compartment to compartment from the higher to the lower end of the drum, the raw waste organic material being fed into the digester at the higher end and partially cured compost being withdrawn at the lower end.
The co-composting technology to which the present invention has particular application embodies a fermentation reactor which is employed to accelerate the microbial conversion of solid waste and sewage sludge into a high quality compost. The process has the ability to compost municipal solid waste and sewage together hence the term co-composting, thereby addressing the two principal waste management problems communities will face in the next few decades. An important step in the overall composting process is the curing of the compost to reduce pathogens. During this critical stage it is important that the temperature of the compost be maintained at a fairly precise level for the entire period designated by the U.S. Environmental Protection Agency guidelines in order to achieve the prescribed pathogen destruction. Should external ambient air be used in a cold climate, for example, without prior heating and the concomitant expenditure of energy, the time to achieve the correct temperatures for the process would be extended and more of the microbes which are essential to carrying out the process could be damaged prior to reaching the proper temperature level. It further should be noted that one system of compost curing currently utilized employs open channels having concrete walls spaced eight to ten feet apart adapted to store compost within the channel six to eight feet high. In systems of this general construction the required pathogen reduction can only be achieved at the center of the mass. Temperatures at the upper surfaces and at the walls are many degrees cooler because of no insulation on the top surface.
I have discovered that by closing the heretofore open curing channels by employing a sliding insulated cover over the channel which can easily be rolled out of the way to allow mechanized or manual turning of selected portions of the compost pile, and then quickly returned to its heat insulating position, that improved temperature control within the channel can be achieved with concomitant improved pathogen destruction.
I have also discovered that by providing a closed air circulating system for use in conjunction with the insulated cover that pathogen destruction is further enhanced along with improved odor control. Typical prior art channel composting systems use ambient building air to aerate the compost pile. This causes undue cooling of compost in the bottom of the pile as the air is circulated through the pile thus jeopardizing the attainment of sufficient temperatures to ensure pathogen destruction throughout the pile.
In conjunction with the sliding insulated cover which in effect converts the open channel into a closed vessel, I employ a system of duct work which can be used to capture spent air from the top of the enclosed area of the channel thereby retaining the heat generated within the pile to achieve much better temperature regulation while at the same time releasing less spent air to the building atmosphere where odor removal is often necessary.
The above described system results in achievement of pathogen destruction in accordance with EPA regulations pertaining to closed vessels thereby materially improving the overall operating efficiency of the composting system which heretofore required a much longer compost residence time within the open channels in order to meet EPA standards.